Plated via interposer

ABSTRACT

An interposer with a contact well, a layer that isolates one side of the interposer from the other side, for isolations of off gasses, and an optional elastomeric pad and a hard stop layer. Included is a method of making the interposer.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to interposers, and more particularly relates to an interposer particularly suited to highly miniaturized applications, a method for making the interposer, and an interposer made by a method.

2. Background Information

Interposers are devices used with electrical components to provide an electrical pathway from one electrical component to another. They are typically about the size of a postage stamp and contain arrays of small contacts (each contact being one interposer), which are placed over one electrical component, so that a second electrical component can be placed in contact with the interposer for full electrical contact between the two components. Interposers may be used in a testing situation in which numerous electrical components are to be connected to a testing device. Interposers can also be used in a situation where an electrical component is to be interchangeable or replaceable in order to make replacement easier.

One particular type of contact that needs to be contacted by interposers is a solder ball. This is a generally semi-spherical shaped ball, which forms the electrode of one electrical component, such as a silicon chip. The electrical connections of an interposer need to be able to contact the solder ball, and possibly correct centering problems and also form a good electrical connection in spite of the possibility of a layer of oxidation being present on the solder ball. As solder balls on electrical components have become increasingly smaller, the present technology to make interposers that make a clean and efficient connection to such solder balls has been surpassed. Several ways are utilized to try to make a good connection with solder balls. These include a metal pinch contact, which is like the tips of a very small pair of tweezers fitting around the solder ball. Another variation on this approach is a connection in which the tips that fit around the solder ball are generally shaped like small spoons, and fit the contours of the sides of the solder ball. Another approach to this problem is the metal “y” contact. Still another interposer design to contact the solder ball is by using a contact of elastomeric material with metallic beads suspended in the material, which form a rough surface for contacting the ball. A similar approach is to use a conductive polymer bump mounted on ceramic. Another approach is to form a pocket etched in silicon. Still another approach is to use a metal probe that contacts the solder ball. Such a metal probe can have a pocket into which part of the solder ball can fit.

These approaches have proven to be only partially successful. A problem with them is that the smallest scale that these devices can be built is a scale in which the center of the balls is approximately 0.75 millimeters (mm) apart. This is called a 0.75 mm pitch array. In such an array, the solder balls are approximately 0.35 mm in diameter. As solder balls become smaller, a 0.65 mm pitch array and even smaller pitch arrays are preferable, and an interposer must be designed that can interface with such a pitch array. Current technologies are unable to achieve a good connection with a pitch array of less than 0.75 mm.

One approach to achieving a good connection between the contact pad of an interposer and a solder ball is to have the contact pad include a hollow space, or well, in the center. This well is achieved in the current technology by drilling a hole through the interposer. This results in an interposer with a through hole. On the side of the interposer opposite from the side that contacts the solder ball, an elastomeric pad is often attached to the lower contact surface. The elastomeric material is filled with metallic balls that create an electrical connection. The ability of the elastomeric pad to flex slightly allows the lower surface of the interposer to compensate for co-planarity problems of a chip or testing machine. The problem with this type of an interposer is that the elastomeric material is inevitably subjected to heat. When a chip is being tested, it is tested in an environment of approximately 125 degrees Centigrade. When a chip is under this temperature, its electrical circuitry is tested. Thus the testing equipment must be able to also survive that temperature. In this temperature range, volatile elements of the elastomeric material tend to off gas. As the elastomeric material emits these volatile gases, an interposer with a through hole will allow these off gases to pass through the interposer and contact the solder balls. They can be deposited on the solder ball, and result in poor electrical connections when the chip is used in its final application. Thus, a hole through the interposer is undesirable, but a well on the surface of the interposer that contacts the solder ball is desirable. An elastomeric pad is also desirable.

Therefore, it is an object of the present invention to make an interposer that is suitable for use with the new generation of smaller pitched solder balls. It is a further object of the invention to provide a method for making an interposer that will result in an interposer that operates with small pitched solder ball arrays. It is a further object of the invention to provide an interposer made by a method in which vias are cut with a laser in the insulating layer. The invention also yields an interposer with a contact well, which does not go all the way through, and which has a layer that blocks off gases.

Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.

SUMMARY OF THE INVENTION

The device includes a method of making an interposer, an interposer made by a specified method, as well as an interposer structure. The goal of interposers is to be able to contact electrodes that are closer and closer together. At present, the limit of technology is for an interposer to effectively contact electrodes that are 0.75 millimeters apart. This is called a pitch of 0.75, and refers to the distance from the center of one ball feature to the center of an adjacent ball feature. The present invention provides an interposer that can be smaller than a 0.75 pitch, as well as a method of making such an interposer.

The invention is an interposer that includes an upper contact pad and a lower contact pad. The two contact pads are connected by connecting posts that penetrate through an insulative planar layer. The insulative planar layer has vias that are drilled and through which the connecting posts of each interposer extend. The upper contact pad, lower contact pad, and connecting posts are made of copper. An advantage of this design is that the well of the upper contact pad serves to make a good connection with a solder ball type electrode. Since the insulative planar layer separates the upper contact pad and the lower contact pad, the solder ball is protected from any off gassing from the lower contact pad.

An optional feature of the interposer is to include an elastomeric pad attached to either the upper contact pad or the lower contact pad. The elastomeric pad includes metallic granules that contribute to the conductivity of the elastomeric pad. The elastomeric pad is configured so that it is conductive even when not under pressure. It is designed to be compressed by about thirty percent of its length.

Another optional feature for the interposer is to include an expansion-limiting device around each of the elastomeric pads. The expansion-limiting device can be a hard stop layer. The hard stop layer is a layer that has holes corresponding to each elastomeric pad. The elastomeric pads protrude through the holes in the hard stop layer. When pressure is applied to the elastomeric pads, the hard stop layer allows the elastomeric pad to expand, but limits their expansion. The hard stop layer can be constructed so that each of the hard stop passages when fitted around the elastomeric pad leaves a small gap between the hard stop material and the elastomeric pad. The upper and lower contact pads of the interposer can be coated with a conductive metal. This is typically nickel, and the nickel may be itself coated with a conductive metal such as gold.

The upper contact pad has an exterior form, which surrounds a well area. The top pad can be different then the bottom pad in shape, size and position. This shape helps create a better connection between ball electrodes and the upper contact surface. The ball electrode partially protrudes into the well, and is contacted by the inside edges of the upper contact pad body. These sharp edges tend to scrape away or penetrate any oxidation that might be present on the ball electrode. They also serve as a self-centering mechanism with the solder ball electrode, so that if the solder ball electrode is off in alignment slightly from the upper contact pad, the well and the inner edges of the upper contact pad help to realign the ball for positive connection.

The interposer of the invention may be made by the following method or process. First, an insulative layer is provided. The insulative layer has a layer of copper bonded to both the top and bottom surfaces. The next step involves using a laser to drill vias through the insulative layer and through the layers of copper. The vias are positioned so that they will serve to connect the upper and lower contact pads of the interposer at a later date. The next step is to coat all surfaces of the insulative layer with a layer of copper, which is typically added by a chemical coating method or by electrolysis. This forms a thin layer of copper over the inside of the vias and over the top surfaces of the upper and lower copper layer.

The next step is to add a layer of photoresist to both the upper and lower surfaces of the insulative layer. The next step is to use artwork to selectively expose the photoresist layers to light, using conventional masking techniques. This artwork exposes the photoresist around an upper contact pad and the central well. It also exposes the photoresist that surrounds the lower contact pad. The lower and upper contact pads are positioned on opposite sides of the insulative sides from each other so that the vias connected the upper. contact pad and the lower contact pad join the two together. Laser Direct Imagining may also be used. LDI is a method that uses a laser to expose the photo resist without using artwork. This will save a step of producing the file and is more accurate. The next step is to chemically develop the photoresist so that the areas that are unexposed to light are chemically removed. If the LDI were used, this would remove areas that are exposed to light. This step removes the upper contact pad and the lower contact pad areas of each interposer. Although this method describes the making of one interposer, it is to be understood that typically a number of interposers will be formed on one unit of insulative layer. This unit of insulative layer can be a small piece, roughly the size of a postage stamp, on which may be mounted numerous interposers. Thus, the term “interposer” refers to a single interposer as well as to an array of interposers on a unit of insulative material.

The next step involves depositing a layer of copper in those areas that are not covered by photoresist. This results in building up the upper and lower contact pads of each interposer and filling the vias between them with copper. This forms the copper body of one or more interposers on the insulative layer.

The next step is to deposit a layer of nickel on the exposed copper surfaces on the upper and lower surfaces. Since the only exposed copper surfaces on the upper and lower surfaces are the contact pads of the interposer, the contact pads of all of the interposers become coated with a thin layer of nickel.

The next step involves depositing another layer of conductive material on the exposed nickel surfaces. This results in the top and bottom surfaces of the interposers being covered with gold. Other conductive materials could also be utilized to enhance durability and conductivity of the interposers, such as Cobalt.

The next step is to remove the upper and lower layers of photoresist.

After removing the photoresist, the areas of exposed copper are also removed. This removes the copper layers on the top and the bottom of the insulative layer, which were the original layers of copper on the insulative layer.

An optional step to this process is to add a layer of elastomeric material to the upper or lower surface of the interposer. The elastomeric material is filled with suspended metallic granules, which are conductive and form numerous conductive pathways through the elastomeric material.

An array of interposers made by the method described and mounted on an insulative layer can be made, and is the typical form this product takes. The upper contact pad is shown as having an interior chamber or well, which is generally octagonal. This interior chamber can also take many other shapes such as square, triangular, star shaped, hexagonal and circular. Each of these shapes would serve the function of helping to center the solder ball electrode and to scrape oxidation or to penetrate the oxidation. They may be generally square in outer dimensions. The outer dimension of these interposers can also take other shapes, including square, rectangular, cross shaped, star shaped, round, hexagonal, octagonal, etc. The upper contact pad can have a different shape than the lower contact pad.

Further, the purpose of the foregoing abstract is to enable the United States Patent and Trademark Office and the public generally, and especially the scientists, engineers, and practitioners in the art who are not familiar with patent or legal terms or phraseology, to determine quickly from a cursory inspection the nature and essence of the technical disclosure of the application. The abstract is neither intended to define the invention of the application, which is measured by the claims, nor is it intended to be limiting as to the scope of the invention in any way.

Still other objects and advantages of the present invention will become readily apparent to those skilled in this art from the following detailed description wherein we have shown and described only the preferred embodiment of the invention, simply by way of illustration of the best mode contemplated by carrying out my invention. As will be realized, the invention is capable of modification in various obvious respects all without departing from the invention. Accordingly, the drawings and description of the preferred embodiment are to be regarded as illustrative in nature, and not as restrictive in nature.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a solder ball with various prior art means of making electrical contact with the solder ball.

FIG. 2 is a view of prior art mechanical contacts

FIG. 3 is a view of prior art mechanical contacts.

FIG. 4A is a view of prior art mechanical contacts.

FIG. 4B is a view of prior art mechanical contacts.

FIG. 5 is a cross-sectional view of the insulative layer of the invention with the upper and lower surfaces coated with copper.

FIG. 6 is a cross-sectional view showing the step in the process of drilling vias with a laser.

FIG. 7 shows the step of coating the insulative layer and the vias with copper electrolyte.

FIG. 8 shows the step of coating the upper and the lower surfaces of the insulative layers with photoresist.

FIG. 9 shows the step of removing the unexposed photoresist portions.

FIG. 10 shows adding copper to vias and contact pad areas.

FIG. 11 shows the step of depositing a layer of nickel to the exposed copper.

FIG. 12 shows the step of depositing a layer of gold to the exposed copper.

FIG. 13 shows the step of removing the layers of photoresist from the upper and lower surfaces.

FIG. 14 shows the step of removing the exposed copper surfaces.

FIG. 15 shows the step of adding a stencil.

FIG. 16 shows the step of adding elastomeric material.

FIG. 17 shows an interposer of the invention with an elastomeric pad.

FIG. 18 shows an interposer of the present invention with a hard stop layer.

FIG. 19 shows an array of interposers positioned on an insulative layer.

FIG. 20 is a side view of an interposer with an elastomeric pad.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

While the invention is susceptible of various modifications and alternative constructions, certain illustrated embodiments thereof have been shown in the drawings and will be described below in detail. It should be understood, however, that there is no intention to limit the invention to the specific form disclosed, but, on the contrary, the invention is to cover all modifications, alternative constructions, and equivalents falling within the spirit and scope of the invention as defined in the claims.

FIG. 1 shows a number of concepts that have been utilized or proposed for contacting solder balls in a small array. These include a metal pinch contact, which is like the tips of a pair of tweezers gripping the sides of a metal ball. Another is a metal “y” contact that accomplishes about the same thing. The other figures show actual or proposed contact methods.

A problem with the mechanical connections shown in FIG. 1 is that when the solder balls become extremely close together, there is not enough room for these mechanical connections to work.

FIG. 2 shows four solder ball electrodes arranged in a 0.75 mm pitch. The mechanical contacts are shown surrounding two of the solder balls. In this arrangement, there is sufficient space for the mechanical contacts.

FIG. 3 shows that there is even more space if the mechanical contacts are arranged to contact the balls in a diagonal fashion. However, FIGS. 4A and 4B show that when the solder balls are at a pitch smaller than 0.75, the mechanical connections do not have sufficient clearance to operate. Actuation at a forty-five degree angle to the axis of the array somewhat improves the situation, but mechanical contacts are problematic. For one thing, they have to be extremely accurate, and the solder balls also have to be equally accurate, so that each mechanical contact can make a perfect connection with each solder ball. This requirement for perfect alignment makes mechanical connections increasingly problematic as the pitch between solder balls becomes smaller.

The interposer of the invention is shown in cross-section in FIG. 14 and is designated as 10 throughout the figures. It includes an insulative layer 12, and an upper contact pad 14 and a lower contact pad 16. The upper and lower contact pads 14 and 16 are connected to each other by a number of connecting posts 18. The connecting posts 18 extend through vias 20, which are laser cut passages through the insulative layer 12. An optional feature of the interposer 10 is a layer of conductive material 22, which is preferably a layer of nickel. An optional feature is a second layer of conductive material, which is preferably a very thin layer of gold 24.

FIG. 17 shows another version of the interposer, which includes an elastomeric pad 26. The elastomeric pad 26 is made of a conductive elastomer. One method of making the elastomer conductive is to embed within it a large number of metallic granules 28.

FIG. 19 shows individual interposers 10 that form the interposer array 30. One feature of the interposer is a well 32 formed within the upper contact pad as shown in FIG. 17. The upper contact pad 14 includes an outer side wall 34, an inner side wall 36, and a wall interface 38, which is the interface between the inner side wall 26 and a top surface 40. The wall interface 38 provides a contact edge that, along with the well 32, accomplishes two functions. The wall interface 38 tends to cut through oxidation on a solder ball, which results in good conductivity in this contact. The wall interface 38 combined with the well 32 also helps to center the solder ball or other electrode accurately on the interposer 10. If a solder ball is not in perfect alignment with the interposer 10, the downward pressure of the connection causes the interposer 10 to deflect its position slightly on the insulative layer 12, and provide a better fit for the solder ball or other electrode. The elastomer pad 26 also aids in compensating for contacts that are not perfectly coplanar.

FIG. 19 shows three interposers 10 of the invention with the attached elastomer pads 26. Shown in FIG. 19 are the insulative layer 12 and a number of interposers 10 forming an array 30. Each interposer 10 has an upper contact pad 14 and a lower contact pad 16. FIG. 20 shows the interposers in contact with solder balls 42. One of the interposers shown shows the elastomeric pad 26 compressed as it might be in use. Each of these four interposers also shows a solder ball electrode 42 with which it is in contact. The solder ball electrode is not part of the interposer, but is part of an electrical device or chip that the interposer is in contact with.

FIG. 18 shows an optional feature of the device, which is a hard stop layer 44. The hard stop layer is a layer that includes hard stop passages 46, through which the elastomer pad 26 protrudes. The hard stop layer serves to limit the expansion that is possible for the hard stop layer 26, while still allowing the elastomer pad to flex upon contact with an electrode.

For purposes of this description, a mil refers to one thousandths of an inch. The insulative layer used in this method of making an interposer is typically two mils thick. Kapton material K4 is also a material that can be utilized. The layers of photoresist that are bonded onto the copper layers are also two mils thick. The nickel layer, which is bonded on, is 250 microns thick. The gold layer is twenty microns thick. By the photoresist layer being two mils thick, when the voids are filled and the photoresist layer is stripped off, that leaves interposer features that are two mils high.

A preferred mode of the invention disclosed is an interposer made by the method of the invention. The interposer of the invention is shown in the figures, as well as a way of making the interposer. The interposer is mounted on an insulative layer, and has a contact pad on the upper surface of the insulative layer and a contact pad on the lower surface of the insulative layer. The upper and lower contact pads are connected to each other through a connecting post via of copper, which extends through a via in the insulative layer. The upper and lower contact pads are also made of copper. Both the upper and lower surface can also be coated with one or more layers of conductive metals, such as a layer of nickel, gold, silver, or other conductive materials. It is advantageous to coat the copper surfaces with a layer of nickel followed by a layer of gold. The upper contact pad has an exterior form that surrounds a well area. This shape helps create a better connection between ball electrodes and the upper contact surface. The ball electrode partially descends into the well, and is contacted by the inside edges of the upper contact pad body. These sharp edges tend to scrape away or penetrate any oxidation that might be present on the ball electrode. They also serve as a self-centering mechanism with the solder ball electrode, so that if the solder ball electrode is off in alignment slightly from the upper contact pad, the well and the inner edges of the upper contact pad help to realign the ball for positive connection.

The interposer of the invention may be made by the following method or process. First, an insulative layer is provided as shown in FIG. 5. This is preferably K4 or Kapton, 2 mils thick. The insulative layer has a layer of copper bonded to both the top and bottom surfaces. These are the first and second copper layers 48, 50, and are preferably 0.365 mils thick, also known as “quarter ounce copper”. The next step involves using a laser 52 to drill vias 20 through the insulative layer and through the layers of copper as shown in FIG. 6. The vias are positioned so that they will serve to connect the upper contact pads 14 and lower contact pads 16 of the interposer 10 at a later time. The next step is to coat all surfaces of the insulative layer with a layer of copper, which is typically added by a multi-step chemical process that is known in the industry. This forms a thin layer of copper, a third layer 54, over the inside of the vias and over the top surfaces of the upper and lower copper layer as shown in FIG. 7. This layer 54 is 80 microns thick.

The next step is to add a layer of photoresist 56 to both the upper and lower surfaces of the insulative layer as shown in FIG. 8. The photoresist is 2 mils thick. DSI Technology may also be used to remove photoresist with a laser. The next step is to use artwork to selectively expose the photoresist layers to light, using conventional masking techniques. This artwork exposes the photoresist around an upper contact pad and the central well. It also exposes the photoresist that surrounds the lower contact pad. The lower and upper contact pads are positioned on opposite sides of the insulative sides from each other so that the vias connected the upper contact pad and the lower contact pad join the two together. The next step is to chemically develop the photoresist so that the areas that are unexposed to light are chemically removed, which includes the upper contact pad and the lower contact pad areas of each interposer. This step is shown in FIG. 9. Although this method describes the making of one interposer, it is to be understood that typically a number of interposers will be formed on one unit of insulative layer. This unit of insulative layer can be a small piece, roughly the size of a postage stamp, on which may be mounted numerous interposers. Thus, the interposer 10 refers to a single interposer as well as to an array 30 of interposers on a unit of insulative material.

The next step is shown in FIG. 10 and involves depositing a fourth layer 58 of copper in those areas that are not covered by photoresist. This results in building up the upper and lower contact pads of each interposer and filling the vias between them with copper. The filled vias forms connecting posts 60. This forms the copper body of one or more interposers on the insulative layer.

The next step is to deposit a layer of nickel 62 on the exposed copper surfaces on the upper and lower surfaces, as shown in FIG. 11. This is an optional step, but is preferred. Since the only exposed copper surfaces on the upper and lower surfaces are the contact pads of the interposer, the contact pads of all of the interposers become coated with a thin layer of nickel 62.

The next step shown in FIG. 12 involves depositing another layer of conductive material 64 on the exposed nickel surfaces. This results in the top and bottom surfaces of the interposers being preferably covered with gold. Other conductive materials could also be utilized to enhance durability and conductivity of the interposers, such as cobalt.

The next step is to remove the upper and lower layers of photoresist 56, as shown in FIG. 13. This also exposes the well 32 that is surrounded by the inner side walls 36 of the upper contact pad 14. For certain sizes of interposers, the photoresist 56 in the well 32 may be removed using a laser.

After removing the photoresist, the areas of exposed copper are also removed, shown in FIG. 14. This removes the copper layers on the top and the bottom of the insulative layer, which were the original layers of copper on the insulative layer. This leaves one or more interposers in the configuration, shown in FIG. 14.

An optional step to this process is to add a layer of elastomeric material to the upper or lower surface of the interposer. The elastomeric material is filled with suspended metallic granules, which are conductive and form numerous conductive pathways through the elastomeric material.

The method of adding the elastomeric pad 26 to the interposer 10 starts with the steps shown in FIG. 15. A stencil 66 is applied to the surface to which the elastomeric pad is to be applied. This could be the upper surface or the lower surface, and the lower surface is shown in the figures. The stencil includes stencil passages that correspond to the lower contact pad 16 of each interposer. The stencils are attached to the lower surface of the insulative layer by an adhesive 70.

As shown in FIG. 16, elastomeric material 72 is applied across the surface of the stencil 66 so that the stencil passages 68 are filled with elastomeric material 72. The elastomeric material 72 includes a large number of metallic granules 28, which add to the conductivity of the elastomeric material. The elastomeric material 72 may be scraped or pressed into the stencil passage 68.

FIG. 17 shows the stencil 66 removed, and the adhesive 70 removed from the insulative layer. This leaves an elastomeric pad 26 with enclosed metallic granules 28.

An optional step is shown in FIG. 18. This involves placing a hard stop layer 44 adjacent the insulative layer 12. The hard stop layer 44 includes hard stop passages 46, which correspond to elastomer pads 26. The hard stop layer 44 serves to limit the possible expansion of the elastomeric pad 26 when it is under pressure. A gap is present between the walls of the hard stop passages and the elastomeric material.

FIG. 19 shows an array 30 that includes individual interposers 10.

FIG. 20 also shows a finished array of interposers. This array does not include the hard stop layer, but does include interposers with an elastomer pad 26.

An array of interposers made by the method described and mounted on an insulative layer can be made, and is the typical form this product takes. The upper contact pad is shown as having an interior chamber or well, which is generally octagonal. This interior chamber can also take many other shapes such as square, triangular, star shaped, hexagonal, and circular. Each of these shapes would serve the function of helping to center the solder ball electrode and to scrape oxidation or to penetrate the oxidation. They may be generally square in outer dimensions. The outer dimension of these interposers can also take other shapes, including rectangular, cross shaped, star shaped, round, hexagonal, octagonal, etc. The upper contact pad 14 can be of different size and shape than the lower contact pad 16.

While there is shown and described the present preferred embodiment of the invention, it is to be distinctly understood that this invention is not limited thereto but may be variously embodied to practice within the scope of the following claims. From the foregoing description, it will be apparent that various changes may be made without departing from the spirit and scope of the invention as defined by the following claims. 

1. An interposer, for providing electrical connection between a contact of one electrical component and a contact of another electrical component: an insulative planar layer with an upper surface and a lower surface, which defines a plurality of vias which penetrate said insulative planar layer; an interposer body, comprising an upper contact pad of copper positioned on said upper surface of said insulative planar sheet, with a plurality of connecting posts which fill said vias and which penetrate said insulative planar sheet and are contiguous with said upper contact pad, and contiguous with a lower contact pad of copper positioned on said lower surface of said insulative planar sheet, with said upper contact pad comprising one or more outer sidewalls and one or more inner sidewalls and a top contact surface, with said inner sidewalls defining a well, for centering contact with a contact ball, in which said insulative planer is not penetrated below said well.
 2. The interposer of claim 1 which further includes an elastomeric pad attached to said lower contact surface, in which said elastomeric pad includes metallic granules which conduct electricity through said elastomeric pad, and in which said elastomeric pad is compressible and conductive.
 3. The interposer of claim 2, which further includes an expansion limiter for each elastomeric pad to limit the amount each elastomeric pad may expand under pressure by surrounding said elastomeric pad by a material with less expansion potential.
 4. The interposer of claim 3, in which said expansion limiter is a hard stop layer, attached to said lower surface of said insulative layer, in which said hard stop layer defines one or more passages through which said elastomeric pads extend, with said hard stop layer being less thick than the height of said elastomeric pads, with hard stop layer provided to limit the possible expansion of said elastomeric pads under compression.
 5. The interposer of claim 4 in which said passages of said hard stop layer are configured to provide a gap between a passage side wall and said elastomeric pad, in which gap said elastomeric pad may expand.
 6. The interposer of claim 1 which further includes a contact edge formed by the interface between said inner walls and said top contact surface
 7. The interposer of claim 1 in which said one or more inner side walls form a generally rectangular well.
 8. The interposer of claim 1 in which said one or more outer side walls form a generally rectangular upper contact pad.
 9. The interposer of claim 1 in which said vias are made by laser cutting of said insulative layer.
 10. The interposer of claim 1 in which the distance from the center of one contact pad to an adjacent contact pad is less than 0.75 mm.
 11. The interposer of claim 1 in which the distance from the center of one contact pad to an adjacent contact pad is less than 0.65 mm.
 12. The interposer of claim 1 in which the distance from the center of one contact pad to an adjacent contact pad is less than 0.50 mm.
 13. The interposer of claim 1 which includes one or more layers of conductive metal on one or more of said contact surfaces.
 14. The interposer of claim 12 in which said contact surfaces are coated with a layer of nickel.
 15. The interposer of claim 12 in which said contact surfaces are coated with a layer of gold. 